Adjustable speed motor systems typically involve the use of specially designed electronic motor controllers coupled to the phase windings of a multi-phase motor via connection cables. An exemplary arrangement is illustrated in FIG. 1 where a three phase motor 10 is coupled to an electronic controller 12 by three connection cables 14, 16 and 18. In operation, cables 14, 16 and 18 act as transmission lines for signals which may take the form of voltage waveforms, provided by the electronic controller 12 to the motor 10.
In many systems, the electronic controller 12 controls the speed or torque of the motor 10 through the application of high frequency voltage pulses to the motor via the connection cables 14, 16 and 18. The frequencies of these applied pulses can be quite high and are often in the kilohertz range. Pulse frequencies of the order of 20 kHz are particularly common, as the operation of switching devices at such speeds does not produce audible noise. The high frequency pulses often take the form of voltage waveforms having steep edges where the voltages change abruptly from a relatively low voltage level to a relatively high voltage level (or vice versa) over a short period of time. The rate of change of the applied voltage over time is mathematically represented by the notation dV/dt. The steeper the edges that define the voltage pulses of the voltage waveform, the higher the value of dV/dt.
The nature of the phase windings in most electric motors causes the motor to appear as a highly inductive load to the connection cables 14, 16 and 18 coupling the motor 10 to the controller 12. At the high switching frequencies and high dV/dt of modern controllers, the inductive load of the motor appears, at least transiently, as an open circuit. Accordingly, the application of high frequency voltage pulses with steep edges (i.e., a high dV/dt) to the motor can result in extreme voltage transients at the points where the connection cables 14, 16 and 18 are coupled to the phase windings of the motor 10. In some systems, the magnitude of these transient voltages can rise to nearly twice the magnitude of the applied voltage pulses.
In new motors, the insulating coating (e.g. of enamel) covering the wiring comprising the phase windings is generally sufficient to handle the voltage spikes caused by the application of the high frequency, high dV/dt voltage pulses. Over time, however, the insulating properties of the enamel coating degrade and a point may be reached where it is no longer capable of handling the voltage spikes resulting from the high frequency pulses. In such instances, the failure of the insulating coating can result in a shorting of winding turns when the voltage stresses caused by the high frequency pulses are applied to the phase windings. Experience has shown that this shorting of winding turns occurs most often in the turns physically adjacent the point where the connection cable is connected to the phase windings. These shorted turns can create a closed circuit where circulating currents are produced. These circulating currents give off heat that tends to further degrade the insulating properties of the wire comprising shorted turns and the winding turns physically adjacent the shorted turns. This heat, in turn, results in additional insulation failures, which result in additional shorted turns. A vicious cycle is instigated which usually results in extreme damage to the motor winding and failure of the motor itself.
The problem of voltage stress induced winding failures in motors has been recognized within the motor art and has been addressed, for example, in D. Potoradi et. al, "Transient Overvoltages Caused by Switching of High Voltage Asynchronous Machines and their Distribution in Stator Windings," 2 Proceedings of the International Conference of Electrical Machines 644-49 (Sep. 5-8, 1994) and K. J. Cornick et al., "Steep-fronted Switched Voltage Transients and their Distribution in Motor Windings" 136 IEE Proceedings 45-55 (March 1982).
The problem of voltage stress induced winding failure can occur in any motor system in which high frequency voltage pulses or voltage pulses having a high dV/dt. are applied to a motor. In particular, this problem can appear in AC induction motor systems, permanent magnet motor systems, and reluctance motor systems. To date, efforts to resolve this problem have not been adequately successful. This invention provides a solution to the winding failure problem particularly applicable to reluctance motor systems, although not necessarily limited thereto. The present invention is defined in the accompanying independent claims. Preferred features are recited in the claims respectively dependent thereon.